(1) Field of the Invention
This invention relates to improved agglomerated articles of a modified zeolite which are stable against a hydrothermal reaction and do not readily become coked with the residue of hydrocarbons.
(2) Description of the Prior Art
Zeolites are widely used in the drying processes for separating water in various industries. When an ion-exchanged zeolite of the type A is used for drying a water-containing gas composed mainly of hydrocarbons, a high separability of water from hydrocarbons can be obtained because of a specific pore size thereof, and therefore, this zeolite is especially highly evaluated in this field. In the drying processes using this zeolite, usually two or three columns, the beds of which are packed with this zeolite, are used and a water-containing hydrocarbon gas is circulated through these adsorption beds alternately or in sequence, and the zeolite layer saturated with water is contacted with a drying hot gas to be thereby regenerated. In short, the drying operation is ordinarily carried out according to the adsorption-desorption cycle. Accordingly, it is always necessary in these drying processes that the adsorbing capability of the zeolite not be degraded by a strong hydrothermal reaction to which the zeolite is subjected at the desorptive regeneration step.
It has been considered that degradation of the adsorbing capability of the zeolite as a result of the hydrothermal reaction is due to destruction of the crystal structure of the zeolite (see Japanese Patent Publication No. 3,956/73). Furthermore, it has been admitted that the degradation of the adsorbing capability as a result of thermal dehydration is characterized in that in the initial stage, the adsorption rate is reduced and, finally, deterioration is caused by the clogging of pores. We often encounter these phenomena in research and actual operation.
As one cause of degradation of the properties of zeolites due to the long-term use thereof, there can be mentioned a so-called coking phenomenon in which a gas to be dried is adsorbed in an adsorbent and the components of the adsorbed gas are polymerized or reacted and passages (pores) in the adsorbent are clogged with the resulting residues, such as polymers or carbonaceous materials. The internal diffusion resistance of the adsorbent is increased by this coking phenomenon and the adsorption zone of the adsorbent at the drying step is expanded, resulting in a substantial reduction of adsorbing capability.
A zeolite agglomerated article is ordinarily prepared by adding water to a mixture of a zeolite powder and a binder such as a clay, kneading the mixture, molding the kneaded mixture into a shaped article, and then drying and heating the shaped article which is in a hydrous state. In the so-prepared molded article, an aggregate of the zeolite crystal and the decomposition product of the clay as the binder is formed at the step of heating and sintering the hydrate in the production process. The properties of the zeolite molded article are ordinarily evaluated while both the properties of the zeolite crystal and the properties of the aggregate of the zeolite crystal and the binder are taken into consideration.